The production of high quality articles, particularly photographic, photothermographic, and thermographic articles, consists of applying a thin film of coating solution onto a continuously moving substrate. Thin films can be applied using a variety of techniques including: dip coating, forward or reverse roll coating, wire-wound coating, blade coating, slot coating, slide coating, and curtain coating (see for example L. E. Scriven; W. J. Suszynski; Chem. Eng. Prog. 1990, September, p. 24). Coatings can be applied as single layers or as two or more superposed layers. While it is usually most convenient for the substrate to be in the form of a continuous substrate, it can also be in the form of a succession of discrete sheets.
The initial coating is either a mixture of solvent and solids or a solution and must be dried to obtain the final dried article. While the cost of a coating process is determined by the coating technique, the cost of a drying process is often proportional to the desired line speed (see E. D. Cohen; E. J. Lightfoot; E. B. Gutoff; Chem. Eng. Prog. 1990, September, p. 30). The line speed is limited by the capabilities of the oven. To reduce costs, it is desirable that the removal of solvent from the coating be as efficient as possible. This is generally accomplished by transferring heat to the coated article as efficiently as possible. This is often accomplished by increasing the velocity of the drying gas at the coating surface, thereby increasing heat transfer and solvent evaporation and thus drying the coating more quickly. The resulting turbulent air, however, increases the tendency for defect formation.
The process of applying a coating to and drying that coating on a substrate can inherently create defects, including Benard cells, orange peel, and mottle. Benard cells are defects arising from circulatory motion within the coating after it has been applied (see C. M. Hanson; P. E. Pierce; Cellular Convection in Polymer Coatings--An Assessment, 12 Ind. Eng. Chem. Prod. Res. Develop. 1973, p. 67).
Orange peel is related to Benard cells. Orange peel is most common in fluid coatings which have a high viscosity to solids ratio. This is due to the tendency of such systems to "freeze in" the topography associated with Benard cells upon loss of relatively small amounts of solvent. The topography can be observed as a small scale pattern of fine spots like the surface of an orange peel. The scale of the pattern is on the order of millimeters and smaller.
Mottle is an irregular pattern or non-uniform density defect that appears blotchy when viewed. This blotchiness can be gross or subtle. The pattern may even take on an orientation in one direction. The scale can be quite small or quite large and may be on the order of centimeters. Blotches may appear to be different colors or shades of color. In black-and-white imaging materials, blotches are generally shades of gray and may not be apparent in unprocessed articles but become apparent upon development. Mottle is usually caused by air movement over the coating before it enters the dryer, as it enters the dryer, or in the dryer (see for example, "Modern Coating and Drying Technology," Eds. E. D Cohen, E. B. Gutoff, VCH Publishers, N.Y., 1992; p. 288).
Mottle is a problem that is encountered under a wide variety of conditions. For example, mottle is frequently encountered when coatings comprising solutions of a polymeric resin in an organic solvent are coated onto webs or sheets of synthetic organic polymer substrates. Mottle is an especially severe problem when the coating solution contains a volatile organic solvent but can also occur to a significant extent even with aqueous coating compositions or with coating compositions using an organic solvent of low volatility. Mottle is an undesirable defect because it detracts from the appearance of the finished product. In some instances, such as in imaging articles, it is further undesirable because it adversely affects the functioning of the coated article.
Substrates that have been coated are often dried using a drying oven which contains a drying gas. The drying gas, usually air, is heated to a suitable elevated temperature and brought into contact with the coating in order to bring about evaporation of the solvent. The drying gas can be introduced into the drying oven in a variety of ways. Typically, the drying gas is directed in a manner which distributes it uniformly over the surface of the coating under carefully controlled conditions that are designed to result in a minimum amount of disturbance of the coated layer. The spent drying gas, that is, drying gas which has become laden with solvent vapor evaporated from the coating, is continuously discharged from the dryer.
Many industrial dryers use a number of individually isolated zones to allow for flexibility in drying characteristics along the drying path. For example, U.S. Pat. No. 5,060,396 describes a zoned cylindrical dryer for removing solvents from a traveling substrate. The multiple drying zones are physically separated, and each drying zone may operate at a different temperature and pressure. Multiple drying zones are desirable because they permit the use of successively lower solvent vapor composition. German Pat. No. DD 236,186 describes the control of humidity and temperature of each drying zone to effect maximum drying at minimum cost. Soviet Pat. No. SU 620766 describes a multistage timber dryer with staged temperature increases that reduce the stress within the timber.
Usually, when multiple zones are present in an oven, they are isolated from one another. The coated substrate is transferred between the zones through a slot. In order to minimize the air and heat flow between zones and to be able to effectively control the drying conditions in each zone, this slot typically has as small a cross-section as possible that will still allow the substrate to pass between zones. However, the adjacent zones are in communication with one another through the slot and thus there is typically a pressure difference between zones. Air flows from one zone to another; and since the dimensions of the slot are small, the air gas velocity is high. Therefore the slots between ovens tend to be sources for mottle defects.
U.S. Pat. No. 4,365,423 discloses an apparatus and method for drying to reduce mottle. FIG. 1 shows an embodiment of this invention. The drying apparatus 2A uses a foraminous shield 4A to protect the liquid coating 6A from air disturbances. The foraminous shield 4A is described to be a screen or perforated plate that sets tip a "quiescent" zone above the substrate promoting uniform heat and mass transfer conditions. The shield 4A is also noted to restrict the extent to which spent drying gas, which is impinged toward the liquid coating 6A, comes in contact with the surface of the coating. This method is reported to be especially advantageous in drying photographic materials, particularly those comprising one or more layers formed from coating compositions that contain volatile organic solvents. This apparatus and method has the limitation that it slows the rate of drying.
U.S. Pat. No. 4,999,927 discloses another apparatus and method for drying a liquid layer that has been applied to a carrier material moving through a drying zone and which contains both vaporizable solvent components and non-vaporizable components. FIG. 2 illustrates this apparatus 2B and method. Drying gas flows in the direction of the carrier material 8B and is accelerated within the drying zone in the direction of flow. In this manner, laminar flow of the boundary layer of the drying gas adjacent to the liquid layer on the carrier material is maintained. By avoiding turbulent air flow, mottle is reduced.
Examples of two other known drying apparatuses and methods are shown in FIGS. 3 and 4. FIG. 3 schematically shows a known drying apparatus 2C in which air flows (see arrows) from one end of an enclosure to the other end. The airflow is shown in FIG. 3 as being parallel and counter to the direction of travel of the coated substrate (i.e., counter-current). Parallel cocurrent airflow is also known.
FIG. 4 schematically shows a known drying apparatus 2D which involves the creation of impingement airflow (see arrows), that is more perpendicular to the plane of the substrate 8D. The impinging air also acts as a means for floating or supporting the substrate through the oven.
U.S. Pat. No. 4,051,278 describes a method for reducing mottle caused by solvent evaporation in the coating zone. Coating a substrate with reduced mottle, such as coating a composition comprising a film-forming material in an evaporable liquid vehicle onto a flexible web or synthetic organic polymer, is achieved by maintaining at least two of the following at a temperature substantially equivalent to the equilibrium surface temperature of the coated layer at the coating zone: (1) the temperature of the atmosphere at the location of coating; (2) the temperature of the coating composition at the location of coating; and (3) the temperature of the substrate at the coating zone. The equilibrium surface temperature is defined as the temperature assumed by the surface of a layer of the coating composition under steady state conditions of heat transfer following evaporative cooling of the layer at the coating zone. After coating, drying of the coated layer is carried out by conventional techniques. This invention includes methods of drying while preventing mottle formation by controlling temperature (i.e., by cooling) at the coating zone and does not address temperature control or mottle formation within the drying oven. Furthermore, this method would be useful only for coatings that cool significantly due to evaporative cooling which subsequently causes mottle.
U.S. Pat. No. 4,872,270 describes a method of drying latex paint containing water and one or more high boiling organic solvents coated onto a carrier film. The process yields a dried paint layer free of blisters and bubble defects. The coated film is passed continuously through a series of at least three drying stages in contact with warm, moderately humid air and more than half of the heat required for evaporation is supplied to the underside of the film. Drying conditions in at least each of the first three stages are controlled to maintain a film temperature profile which causes the water to evaporate at a moderate rate but more rapidly than the organic solvents, thus achieving coalescence of the paint and avoiding the trapping of liquids in a surface-hardened paint layer. Bubble formation is reportedly eliminated by controlling the vapor pressure of the volatile solvent within the film. The formation of mottle occurs due to a different mechanism than blisters and requires different methods for control and elimination.
U.S. Pat. No. 4,894,927 describes a process for drying a moving web coated with a coating composition containing a flammable organic solvent. The web is passed through a closed-type oven filled with an inert gas and planer heaters on top and bottom of the web. The coating surface is reported to be barely affected by movement of the inert drying gases due to the small amounts of gas required. No discussion of the criticality of the gas flow system or of the need to prevent mottle is given.
U.S. Pat. No. 5,077,912 describes a process for drying a continuously traveling web coated with a coating composition containing an organic solvent. The coating is first dried using hot air until the coating is set-to-touch. It is sufficient that the drying conditions, such as temperature and hot air velocity, are adjusted so as to obtain the set-to-touch condition. Set-to-touch corresponds to a viscosity of 10.sup.8 to 10.sup.10 poise. Residual solvent is then removed using a heated roll. This method is said to reduce drying defects, decrease drying time, and reduce oven size. No discussion on the construction of the oven, methods of drying, or the criticality of the gas flow system and path is given.
U.S. Pat. No. 5,147,690 describes a process and apparatus for drying a liquid film on a substrate which includes a lower gas or air supply system and an upper gas or air supply system. Heated gas on the underside of the substrate forms a carrying cushion for the substrate and at the same time supplies drying energy to the substrate. The exhaust air is carried away through return channels. Slots for the gas supply and return are arranged alternately in the lower gas system. The upper gas or air supply system has a greater width than the lower gas or air supply system. In the upper gas or air supply system, the supply air or gas is diverted by baffles onto the substrate and returned over the substrate web as return air or gas. The upper gas or air supply system is subdivided into sections for the supply air and exhaust air, each section includes two filter plates of porous material.
U.S. Pat. No. 5,433,973 discloses a method of coating a magnetic recording media onto a substrate, wherein the coating is substantially free of Benard cells. The method comprises the steps of: (a) providing a dispersion comprising a polymeric binder, a pigment, and a solvent; (b) coating the dispersion onto the surface of a substrate; (c) drying the dispersion; (d) calculating values comprising .mu., .beta., and d representing the viscosity, temperature gradient, and wet caliper of the dispersion respectively; and (e) during the course of carrying out steps (a), (b), and (c), maintaining the ratio ##EQU1## below a threshold value sufficient to substantially prevent the formation of Benard Cells in the magnetic recording media coating. No discussion of the interior of the drying oven and arrangement of air inlets and exhausts is given.
A number of methods involve the control of the drying gas within the oven. For example, U.S. Pat. No. 5,001,845 describes a control system for an industrial dryer used to remove a flammable solvent or vapors from a traveling web of material. Sensors within each zone measure the oxygen content of the pressurized atmosphere. If the oxygen content exceeds a given limit, an inert gas is added. At the same time, the pressure is maintained within the oven body by releasing excess gas to the atmosphere.
U.S. Pat. No. 5,136,790 describes a method and apparatus for drying a continuously moving web carrying a liquid, wherein the web is passed through a dryer in which the web is exposed to a recirculating flow of heated drying gas. Exhaust gas is diverted and discharged from the recirculating gas flow at a gas velocity which is variable between maximum and minimum levels, and makeup gas is added to the recirculating gas flow at a gas velocity which is also variable between maximum and minimum levels. A process variable is sensed and compared to a selected set point. A first of the aforesaid flow rates is adjusted to maintain the process variable at the selected set point, and a second of the aforesaid flow rates is adjusted in response to adjustments to the first drying gas velocity in order to insure that the first drying gas velocity remains between its maximum and minimum levels. No discussion of the interior of the drying oven and arrangement of air inlets and exhausts is given.
Soviet Pat. No. SU 1,276,889 describes a method for controlling drying gas by controlling the air gas velocity within the oven. In this method, fan speed in one zone is adjusted, controlling the air flow rate, in order to maintain the web temperature at the outlet to a specified temperature. This approach is limited in that increasing the air gas velocity in order to meet a drying specification can lead to mottle.
The physical state of the drying web can also be used to control the drying ovens. For example, in Soviet Pat. No. SU 1,276,889, noted above, the temperature of the web at the outlet of the oven was used to set the air flow rate.
U.S. Pat. No. 5,010,659 describes an infrared drying system for monitoring the temperature, moisture content, or other physical property at particular zone positions along the width of a traveling web, and utilizing a computer control system to energize and control for finite time periods a plurality of infrared lamps for equalizing physical property and drying the web. The infrared drying system is particularly useful in the graphic arts industry, the coating industry and the paper industry, as well as any other applications requiring physical property profiling and drying of the width of a traveling web of material. No discussion of the interior of the drying oven and arrangement of air inlets and exhausts is given.
U.S. Pat. No. 4,634,840 describes a method for controlling the drying temperature in an oven used for heat-treating thermoplastic sheets and films. A broad and continuous sheet or film is uniformly heated in a highly precise manner and with a specific heat profile by using a plurality of radiation heating furnaces, wherein in the interior of each radiation heating furnace, a plurality of rows of heaters are arranged rectangularly to the direction of delivery of the sheet or film to be heated. A thermometer for measuring the temperature of the sheet or film is arranged in the vicinity of an outlet for the sheet or film outside each radiation heating furnace. Outputs of heaters arranged within the radiation heating furnaces located just before the respective thermometers are controlled based on the temperatures detected by the respective thermometers by using a computer.
Two other patents address drying problems, but fail to address the problem of mottle. U.S. Pat. No. 3,849,904 describes the use of a mechanical restriction of air flow at the edge of a web. Adjustable edge deckles are noted as forming a seal with the underside of a fabric allowing for different heating conditions to occur at the edge. This allows the edge of the fabric to be cooled while the remainder of the fabric is heated. This approach, however, is not advantageous when a polymer substrate is used. Possible scratching of the polymer substrate can generate small particulates which can be deposited on the coating. U.S. Pat. No. 3,494,048 describes the use of mechanical means to divert air flow at the edge of the web. Baffles are noted as deflecting air and preventing air from penetrating behind paper in an ink dryer and from lifting the paper from a drum. Keeping the paper on the drum prevents the drying ink from being smeared.
A need exists for a drying apparatus and method which reduces, if not eliminates, one or more coating defects such as mottle and orange peel, yet permits high throughput. In addition to the drying of coatings used to make photothermographic, thermographic, and photographic articles, the need for improved drying apparatus and methods extends to the drying of coatings of adhesive solutions, magnetic recording solutions, priming solutions, and the like.